In most electrochemical reactions, including all such reactions in which an aqueous electrolyte is used, hydrogen is generated either as a principal product of the reaction or as the result of a parasitic reaction. Conventionally, such generated hydrogen is periodically or continuously purged from the electrochemical system.
Many electrochemical reactions are pH dependent, at least to some extent. For example, either the reaction itself or the efficiency of the reaction may be dependent on the pH. When an acid pH is desired, it is conventional to add an appropriate acid to the electrolyte or, if the electrolyte inherently has the desired pH, to continuously conduct the reaction with fresh electrolyte in order to maintain the desired acid value.
One well known electrochemical reaction is the electrolytic dissociation of an aqueous zinc chloride electrolyte into zinc and chlorine. This reaction has been utilized in a secondary battery system as described in Symons U.S. Pat. No. 3,713,888, which is hereby incorporated by reference. During the charging of the secondary battery, zinc plates out on one electrode and chlorine is generated at the other electrode. Symons then stores the halogen generated for use in discharging of the battery system in the form of a halogen hydrate.
During operation of the Symons battery system, hydrogen is coproduced during the charging of the battery. The gas has been periodically or continuously purged. Such purging has been found to be undesirable, however, because chlorine is also lost along with the hydrogen and has to be replaced, the lost chlorine also presents a disposal problem, and the loss of hydrogen from the system increases the pH of the electrolyte. The pH increase had a number of particularly undesirable consequences. For example, when the pH increased to greater than 2.5, the morphology of the deposited zinc was adversely affected resulting in a gray, spongy material in contrast to the dense crystaline zinc formed at a pH of 0-1.5. The spongy zinc tends to grow faster and develop dendrites earlier than the crystalline deposit, thereby reducing the coulombic as well as the energy storage capacity of the battery system. Additionally, as the pH rises, the hydroxyl ion concentration also rises making it easier to discharge OH.sup.- at the chlorine electrode and to generate oxygen, a significant portion of which immediately oxidizes the graphite electrode used. The graphite grain boundaries are especially susceptible to attack, resulting in erosion and disintergration of the electrode structure and thereby drastically reducing the life of the chlorine electrode. It was apparent that there was a need for a means to conveniently maintain and control the pH of the electrolyte.
It is the object of this invention to provide a simple, convenient and effective means of maintaining the pH of an electrolyte in an electrochemical system at a desired level. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.